Heat the oven to 350°F. Spray a 12-inch pizza pan with the cooking spray.
Heat the oil in 12-inch skillet over medium heat. Add the onion and mushrooms and cook until they're tender and the liquid is evaporated, stirring occasionally. Remove the skillet from the heat.
Stir the egg, milk, pasta and 1/2 cup cheese in a medium bowl. Spread the pasta mixture on the pizza pan.
Bake for 20 minutes. Spread the sauce over the pasta crust. Top with the onion mixture. Sprinkle with the remaining cheese.
Bake for 15 minutes or until the cheese is melted. Let stand for 5 minutes before slicing.
Serving Suggestion: Serve with a mixed green salad with walnuts and red wine vinaigrette. For dessert serve fresh fruit salad.